The present invention relates to a method for producing a silicon solar cell with a back-etched, preferably selective emitter as well as a corresponding solar cell.
It is known that emitters produced at a surface of a solar cell often exhibit, for production-related reasons, a high doping concentration directly at the surface. This high doping concentration can lead to recombination losses, especially with respect to charge carrier pairs generated close to the surface.
It may therefore be desirable to make available a production method for a solar cell, wherein the doping concentration at the surface of the emitter can be reduced in a technologically straightforward manner.
For the most part, the solar cells currently manufactured industrially are produced based on silicon, especially crystalline silicon. The vast majority of these solar cells are provided with a full-area homogeneous emitter layer at the front-side surface and/or at the rear-side surface of the solar cell substrate. The metal contacts are produced by means of thick-film pastes in the screen-printing process in the case of many of the silicon solar cells industrially produced nowadays. For this purpose, a metal-particle-containing paste is printed locally onto the front-side emitter and then fired into the emitter, in order to form a good electrical contact with the emitter layer.
It is known here that it may be necessary to provide the emitter layer, at least in the zones contacted by the metal contacts, with a high doping concentration in the region of the emitter surface in order to obtain a good ohmic contact.
A characteristic parameter for assessing the quality of the emitter, i.e. the doping concentration integrated over the cross-section of the emitter layer, is the so-called sheet resistance. The greater the sheet resistance, the smaller the doping concentration inside the emitter layer and the smaller, as a rule, the doping concentration at the surface of the emitter layer. It has been found that, with conventionally produced emitters, a maximum sheet resistance of emitters capable of being contacted with screen-printing metallisation techniques typically lies in the range from 50-60 ohms per square. Emitter layers with higher sheet resistances and thus fundamentally lower doping usually can no longer be contacted reliably by means of thick-film pastes.
When using industrially advantageous screen-printing metallisation techniques, it is therefore necessary to make available emitter layers with a high surface doping concentration in the region of the metal contacts. On the other hand, however, it is known that such a high surface doping concentration can be accompanied by heavy recombination losses at the surface of the solar cell. In particular, charge carrier pairs which are produced by high-frequency (blue or UV) light very close to the front-side solar cell surface recombine rapidly inside this strongly doped emitter layer and can therefore no longer contribute to the solar cell current. This can reduce the IQE (internal quantum efficiency) in the high-frequency light spectrum and thus the total current supplied by the solar cell, which ultimately reduces the efficiency of the solar cell. An additional effect of a high surface doping concentration may be a so-called band gap narrowing, which can lead to a reduced open circuit voltage. Attempts to meet these contradictory requirements for good contactability on the one hand and a high IQE on the other hand led to the concept of the so-called selective emitter. In the case of the latter, the emitter zones directly beneath the metal contacts are strongly doped locally, whereas the zones lying in between have a much lower doping concentration.
Several methods for producing selective emitter structures have already been developed and tested, mainly on the laboratory scale. In one approach, a selective emitter structure can be produced by two separate diffusion processes in two separate process steps using a local masking layer, for which dielectric layers are often used. Here, however, there is the need for a plurality of high-temperature diffusion processes, and this can increase the production cost significantly. Alternatively, a selective emitter structure can be produced by local etching of an emitter layer previously produced homogeneously.
However, such production methods are often not compatible with the other process steps currently used industrially, such as for example the screen-printing metallisation. Furthermore, problems can occur in the sense that the doping concentration is locally inhomogeneous in the individual emitter zones due to a non-uniform etching process.
Previous approaches at producing silicon solar cells with a selective emitter using two diffusion processes have usually been technically expensive and scarcely able to be implemented industrially on account of their high cost. On the other hand, the production of a selective emitter structure by local etching of the emitter has for the most part only been achieved in the laboratory hitherto, methods chiefly having been tested in which etching of the emitter has been carried out after a metallisation of the solar cells. These production methods have usually led to a considerable decline in the efficiency of the solar cells or were scarcely able to be implemented on an industrial scale.